1. Field of the Invention
The invention concerns a method for joining two objects and a corresponding joining element being applicable to the method, wherein the joining element consists at least partly of a material having thermoplastic properties and the method comprises the application of mechanical vibration, e.g. ultrasonic vibration.
2. Description of Related Art
Publications WO 98/42988 and WO 00/79137 describe joining elements which consist at least partly of a material with thermoplastic properties and which are, for example, pin-shaped. Such a joining element is anchored in an object of a porous material by positioning it in a blind hole provided in the object or by forcing it through the surface of the object and by then applying mechanical vibration (e.g. ultrasonic vibration) to the joining element while simultaneously pressing it further into the object. Due to the mechanical vibration, the thermoplastic material of the joining element is liquefied at the surface of the joining element where this surface is pressed against the porous material and, in its liquid state is pressed into the pores of the porous material where it forms an anchorage on re-solidification. In the anchorage area, the porous material is interpenetrated by the thermoplastic material such that it constitutes an intricate positive connection between the two materials. Application of the mechanical vibration is effected by positioning a vibrating element, e.g. the sonotrode of an ultrasonic device, on the joining element (e.g. on the end of a pin-shaped joining element), where it protrudes from the object in which it is to be anchored.
Publication WO 96/01377 describes a method for joining two objects of a porous material with the aid of a dowel-shaped joining element consisting at least partly of a material having thermoplastic properties and with the aid of mechanical vibration. In each one of the objects to be joined, a blind hole is provided and one end of the joining element is positioned in each one of the blind holes. The two objects are then pressed towards each other in a direction which is substantially parallel to a line connecting the two ends of the joining element and, simultaneously, one of the objects is excited with mechanical vibration.
According to WO 96/01377 the two blind holes and the two ends of the joining element are similar and the length of the joining element corresponds substantially to the total depth of the two blind holes. The two ends of the joining element and the two blind holes provided in the two objects are conical and the joining element and blind holes are dimensioned such that when positioning the joining element in the blind holes, the joining element does not reach to the bottom of the blind holes and therefore there remains a gap between the two objects. The joining action is carried out by positioning the assembly of the two objects and the joining element between a non-vibrating support and an element which is capable of being excited to mechanically vibrate, for example, the sonotrode of an ultrasonic device. The excitable element and the support are forced towards each other by e.g. pressing the excitable element onto one of the objects, and simultaneously, the excitable element is vibrated. On pressing and vibrating, the thermoplastic material of the joining element is liquefied where it is in contact with the walls of the blind holes and the joining element is forced deeper into the blind holes, whereby the gap between the parts is reduced. As soon as the gap is closed, no more pressure can be applied to the joining element and the joining action is therefore finished. The thermoplastic material which, during vibration, is liquefied infiltrates into the porous object material and anchors the joining element in the two objects.
With the above briefly described joining method according to the publication WO 96/01377 satisfactory results can be achieved in very specific cases only. In order to be applicable in a more general manner, it needs improvement.